This invention relates to a porous tantalum capacitor electrode and more particularly to such an electrode made by electroplating a high-surface-area tantalum sponge on only a portion of a tantalum lead.
The usual method for making porous tantalum capacitor electrodes includes coating tantalum powder particles with an organic binder, dispensing the coated powder into a mold press cavity, compressing the powder to produce a pellet that is held together by the binder, heating the pellet under vacuum to remove the binder and vacuum sintering the pellet at least one time to give it mechanical strength and to purge it of impurities such as carbon and nitrogen. Even these drastic measures, however, cannot remove any oxygen it may have acquired. A tantalum lead wire may be welded to the pellet and sintered again; or the tantalum lead wire, often referred to as a riser wire, may be partially inserted into the mold cavity so that the powder is compressed about a portion of it. A more detailed description of methods for making porous anodes and making capacitors from them are provided by Rogers in U.S. Pat. No. 3,627,520 issued Dec. 14, 1971, by Bernard in U.S. Pat. No. 4,059,442 issued Nov. 22, 1977, by Shirn et al. in U.S. Pat. No. 4,127,680 issued Nov. 28, 1978 and by Pellerin et al. in U.S. Pat. No. 4,025,827 issued May 24, 1977. These patents are assigned to the same assignee as is the present invention.
The tantalum powder is usually obtained by reacting a mixed alkali-tantalum-halide salt, most commonly K.sub.2 TaF.sub.7 known as K-salt, with metallic sodium that results in a tantalum precipitate. The mixed halide salts can be washed away leaving only the tantalum powder. Most if not all commercial tantalum refining produces such a mixed alkali-tantalum-halide salt.
These steps taken together constitute an effective but high cost process for making tantalum powder. Since metallic sodium is electrochemically refined and owing to the high surface area of the precipitated tantalum, these processes are especially expensive because they are power intensive and time consuming.
A less power intensive way to obtain metallic tantalum has been known for a score of years, namely its direct electrodeposition from a fused alkali-tantalum-halide salt. Substantial effort and attention has been directed toward the avoidance in this process of forming dendritic deposits and toward obtaining solid coherent deposits. It has been established by numerous independent workers in this field that the universally sought-after coherent deposits can be electroplated from such K-salts at relatively low current levels, i.e. no more than three hundred mA/cm.sup.2 current density under the most favorable conditions including periodic reversal of the electroplating current. This process, however, would have to be followed by difficult fracturing and comminuting steps to obtain the necessary powder for making a porous tantalum capacitor electrode. The later steps would essentially remove the cost savings that might otherwise have been provided via the electroplating process.
It is known to electroplate metals using aqueous electrolytes at high cathode current densities, e.g. 8 to 36 amperes/dm.sup.2 (80 to 360 mA/cm.sup.2) to plate out nodules that could be periodically or continuously removed in the form of metal powder. Such nodules are relatively easy to remove and have shapes approaching spherical particles that is desirable for powder particles. However, tantalum and niobium powders have not been produced in this manner so far as is known probably because aqueous salts of these metals are difficult to acquire. It is also known to electroplate metals using nonaqueous fused-salts as the electrolyte for plating refractory metals such as tantalum. Such a plating method is comparatively difficult and expensive and has been applied only for producing coherent coatings of refractory metals. Such plating requires direct current (DC) densities typically below 250 mA/cm.sup.2 but may reach 375 mA/cm.sup.2 peak when the current is periodically reversed.
It is an object of this invention to circumvent some of the high-cost elements of the prior art and provide a relatively simple and low cost method for making a porous tantalum capacitor electrode.